International Seminar on Aerospace Science And Technology III DIGITAL COMMUNICATION BETWEEN GROUND STATION USING RASPBERRY PI Sonny Dwi Harsono Center for Satellite Technology, National Institute of Aeronautics and Space Jl. Cagak Satelit Km 04 Rancabungur, Bogor, Indonesia 16310 Email : [email protected] Abstract Ground Station or Earth Station has a very important role in performing TT&C for satellite, in addition to that the communication between ground station is also very important for the coordination of operation and data collection from the satellites. In the event of a disaster at the location where the earth station is there, will might be disruption of coordination between earth stations due to lack of Internet as a communication link between ground stations. To prepare for these possibilities, I conducted research to find other ways of communicating between a ground stations. Raspberry Pi is a single board computer that is on the rise lately, this device uses minimum energy source, can even using a battery. This research involves a several devices, among others transceiver VHF / UHF, antenna VHF / UHF, open source software fldigi, interfacing and the relationship between such devices to make another scenario for solve this problem. This research shows that digital communication between Ground Station can be make via radio frequency without using connection from internet, ie using a Raspberry Pi and radio to send or receive a digital data from each Ground Station. Key Words: Ground Station, Digital Communication, Raspberry Pi, Fldigi 1. INTRODUCTION The tracking, telemetry and command (TT&C) subsystem monitors and controls the satellite right from the lift-off stage to end of its operational life in space. The tracking part of the subsystem determines the position of the spacecraft and follows its travel using angle, range and velocity information. The telemetry part gathers information of the health of various subsystems of the satellite. It encodes this information and then transmits the same towards the ground station. The command element receives and executes remote control commands from the control center on earth to effect change to the platform functions, configuration, position and velocity. The TT&C subsystem is therefore very important, not only during orbital injection and the positioning phase but also throughout the operational life of the satellite. Figure 1-1 shows the block schematic arrangement of the basic TT&C subsystem, tracking is used to determine the orbital parameter of the satellite on a regular basis. This help in maintaining the satellite in the desired orbit and in providing look-angle information to the ground station. Angle tracking can be used to determine the azimuth and elevation angle from the ground station. The time interval measurement technique can be used for the purpose of ranging by sending signal via a command link and getting a return via the telemetry link. The rate of change of range can be determined either by measuring the phase shift of the return signal as compared to that of transmitted signal or by using a pseudorandom code modulation and the correlation between transmitted and received signal During the orbital injection and positioning phase, the telemetry link is primarily used by the tracking system to establish a satellite to earth station communication channel. After the satellite is put into desire slot in its intended orbit, its primary function is to monitor the health of various other subsystems on board the satellite. It gathers data from variety of sensors and then transmits that data to the ground station. The data include a variety of electrical and nonelectrical parameters. The sensor output could be analogue or digital. Wherever necessary the analogue output is digitized, with the modulation signal as digital, a various signals are multiplexed using the time division multiplexing technique. Since the bit rates involved in telemetry signal are low, a smaller receiver bandwidth with a good signal-to-noise ratio is used at the end of ground station. The most visible element of a Ground Station is the outdoor antenna, which can take many forms and sizes to correspond to the application requirement (gain, size ) of every Ground Station [2] 34 International Seminar on Aerospace Science And Technology III Figure 1-1 Block schematic of the basic TT&C subsystem [1] 1.1 Raspberry Pi The Raspberry Pi computer is the size of a credit card, completely silent and costs under £30. The operating system runs from an SD flash card, allowing its personality to instantly be switched by swapping cards. Its potential uses are staggering, and as yet, not fully explored, but it has already been tested as a multimedia player with streaming capabilities, a games machine, an internet browser and a hardware development board. It is intended to be used as an educational device for people of all ages and skill levels. The Raspberry Pi is currently available as two different models, known as the Model A and the Model B. While there are differences, with the Model A sacrificing some functionality in order to reduce its cost and power requirements, both share plenty of similarities that you’ll learn about in this chapter. Figure 1-2 shows a Raspberry Pi ver.1 Model B. [5] Figure 1-2 Raspberry Pi Ver.1 Model B [4] In the centre of all Raspberry Pi boards is a square semiconductor, more commonly known as an integrated circuit or chip. This is the Broadcom BCM2835 system-on-chip (SoC) module, which provides the Pi with its general-purpose processing, graphics rendering and input/output capabilities. Stacked on top of that chip is another semiconductor, which provides the Pi with memory for temporary storage of data while it’s running programs. This type of memory is known as random access memory 35 International Seminar on Aerospace Science And Technology III (RAM), as the computer can read from or write to any part of the memory at any time. RAM is volatile, meaning that anything stored in the memory is lost when the Pi loses power or is switched off. Above and below the SoC are the Pi’s video outputs. The silver (bottom) connector is a High Definition Multimedia Interface (HDMI) port, the same type of connector found on media players and many satellite and cable set-top boxes. When connected to a modern TV or monitor, the HDMI port provides high-resolution video and digital audio. The yellow (top) connector is a composite video port, which is designed for connection to older TVs that don’t have an HDMI socket. The video quality is lower than is available via HDMI, and there’s no audio; instead, audio is provided as an analogue signal on the 3.5mm audio jack to the right of the composite video socket. The pins to the top-left of the Pi compose the general-purpose input-output (GPIO) header, which can be used to connect the Pi to other hardware.. The GPIO port is extremely powerful, but it’s fragile; when handling the Pi, always take care to avoid touching these pins, and never connect anything to them while the Pi is switched on. The plastic and metal connector below the GPIO port is the Display Serial Interface (DSI) port, for connecting digitally driven flat-panel display systems. These are rarely used except by professional embedded developers, as the HDMI port is more flexible. A second plastic and metal connector, found to the right of the HDMI port, is the Camera Serial Interface (CSI) port, which provides a high-speed connection to the Raspberry Pi Camera Module or other Pi-compatible CSI-connected camera system. To the very bottom-left of the board is the Pi’s power socket. This is a micro-USB socket, the same type found on most modern smartphones and tablets, switches the Raspberry Pi on; unlike a desktop or laptop computer, the Pi doesn’t have a power switch and will start immediately when power is connected. On the underside of the Raspberry Pi board on the left-hand side is an SD card slot. A Secure Digital (SD) memory card provides storage for the operating system, programs, data and other files, and is non-volatile; unlike the volatile RAM, it will retain its information even when power is lost. The right- hand edge of the Pi will have different connectors depending on which model of Raspberry Pi we have, the Model A or the Model B. Above these is a series of Light Emitting Diodes (LEDs), the top two of which—marked ACT and PWR and providing an activity notification and power notification respectively—are present on all boards. 1.2 Model A The least expensive of the Raspberry Pis, the Model A shown in Figure 1-3 is designed to be affordable yet flexible. As well as its lower cost compared to the Model B, the Model A draws less power and is a good choice for projects that use solar, wind or battery power. Although the Model A’s BCM2835 SoC is just as powerful as the one found on the Model B, it comes with half the memory at 256MB. This is an important consideration when deciding which model to buy, as it can make more complex applications run slowly—in particular, those applications that turn the Pi into a server. Figure 1-3 Raspberry Pi Ver.1 Model A [4] The Model A has only a single port on its right-hand edge, a Universal Serial Bus (USB) port. This is the same type of port found on desktop and laptop computers, and allows the Pi to be connected to almost any USB-compatible peripheral. Most commonly, the USB port is used to connect a keyboard for interacting with the Pi.